Recent clinical research has established that high dairy product intake can significantly reduce body weight and fat mass. Dietary calcium may reduce body weight and fat mass, however, recent studies demonstrate that this effect is enhanced by dairy product intake. Our own studies suggest that dietary calcium intervention increases lipid oxidation in human volunteers and improved vitamin D status is associated with increased energy expenditure. These as well as several other mechanisms have been proposed to mediate the impact of dairy product consumption on body fat mass, including 1) reducing food intake; 2) formation of calcium and fatty acid complexes leading to reduced fat absorption; 3) increasing adipocyte lipolysis; and 4) reducing lipogenesis. These metabolic alterations are potentially mediated by calcium regulated hormones. The objective of these studies is to determine the mechanism(s) by which inclusion of dairy products in the diet induces changes in body composition. Our central hypothesis is that in muscle, increased lipid oxidation is induced by dietary calcium intake and increased energy expenditure is induced by vitamin D intake. We further hypothesize that increased lipolysis from the adipocyte is mediated by dairy product intake. We will address these hypotheses by determining the independent and cooperative effects of the following on overall body mechanisms regulating energy balance: 1) dietary calcium and vitamin D; and 2) dairy product intake with dietary calcium and vitamin D. Identifying the mechanisms by which dairy product intake regulates energy balance will contribute to determining when and how dairy product intake results in changes in body composition. This information would be important in helping to establish dietary recommendations for dairy products or its' components to aid in reducing the growing numbers of obese and overweight individuals and their attendant health problems.